This is according to a new, interesting job listing.
Make Free Calls with Facebook Messenger on Your iPhone
If you’re using the Facebook Messenger iPhone app, you’ll be happy to hear that they are now offering a free calling service for those in the US. The calls are made directly through the app and will work on 3G, 4G and of course, WiFi. It should be noted, however, that this calling service does indeed use VoIP technology and will utilize your data connection.Something to keep in mind while making calls over your network– especially those on a very limited data plan. If you’re on WiFi, you have nothing to worry about.
If you’re on the latest version of the app, the feature should appear without the need of any physical updating through the App Store. With an active conversation open you simply tap the “i” icon in the top-right corner of the screen, hit “Free Call” and you’re good to go.
Source: The Verge